I. Field of the Invention
This invention relates to heat-insulating refractory linings used for aluminum reduction cells and other similar equipment which contains fluorides at high temperatures. More particularly, the invention relates to barrier layers in such refractory linings intended to inhibit or restrict the migration of fluorides from the contents of the cell or other equipment into the refractory lining materials.
II. Background of the Prior Art
Hall-Heroult cells used for the production of aluminum metal from alumina by electrolysis hold a charge of molten electrolyte which contains cryolite (Na.sub.3 AlF.sub.6), a complex fluoride consisting of a mixture of sodium fluoride (NaF) and aluminum fluoride (AlF.sub.3) which is exposed to high temperature (usually higher than 900.degree. C.). The bottom wall (or floor) of the cell has a lining of carbon blocks forming a cathode and one or more layers of refractory material between the cathode blocks and the outer metal shell of the cell. The refractory lining provides heat and electrical insulation for the cell, which is necessary for efficient cell operation.
A problem which is encountered in cells of this kind is that cell contents, particularly the fluoride components, tend to penetrate through the carbon cathode layer into the refractory layers. This causes two particular problems. Firstly, the fluoride may react with the refractory material, normally an alumina-silica based material, to produce a compound of lower melting or glass transition point than the original material, thus causing the refractory to deform or lose its high heat insulating properties. Secondly, the presence of fluorides in the refractory lining creates environmental problems when the refractory linings are eventually to be disposed of. This is because the fluorides, e.g. NaF, are water-soluble and readily leach out of waste refractories disposed of in land-fill sites.
Penetration by the cell contents into the refractory linings can take place in two forms. Firstly, the molten material may penetrate through gaps in the carbon cathode lining and pores within the refractory material promoting their dissolution. However, in modern cells, liquid penetration can often be substantially eliminated by the use of suitable joint compounds and cements for the cathode blocks, so this may not be a major concern. Secondly, solid fluoride produced from reactions in the cathode blocks, as well as metallic sodium which originates from solid diffusion through these blocks may penetrate the refractory lining. Such penetration cannot be stopped merely by avoiding cracks in the cathode lining and it results in considerable fluoride contamination of the refractory layers and deterioration of their insulating properties.
This problem has been addressed in my prior U.S. patent application Ser. No. 07/527,533 filed on May 23, 1990, the disclosure of which is incorporated herein by reference. In this prior application, a layer of alumina-silica based insulating material having a high alumina:silica ratio is positioned immediately below the cathode blocks because material of this kind is not harmed by contact with NaF or metallic Na at high temperatures. Alumina-silica based materials of lower alumina:silica ratio (which are less expensive and may be more highly insulating) are positioned below this first layer, particularly in those regions of the insulation subject to temperatures below 700.degree. C., where reaction with NaF and Na is not problematic (i.e. do not cause glassification of the material). However, while this deals with the harmful effects of NaF and Na on the insulating material, it does not deal with the problem of environmental damage caused by disposal of fluoride-containing refractories.
It would therefore be advantageous to provide a layer of material in the cell wall beneath the carbon cathode layer to act as a barrier against penetration of solid fluoride.
U.S. Pat. No. 4,536,273 which issued on Aug. 20, 1985 to Seltveit discloses the concept of providing a diffusion barrier for the bottom lining of electrolysis cells. The barrier has a composition that, upon reaction with penetrating sodium fluoride-containing melt, forms solid compounds at the operational temperature of the cell. The objective is to cause all of the fluoride penetrating the carbon cathode to become bound as solid CaF.sub.2, which is solid at the temperatures normally encountered below the cathode lining (e.g. 900.degree. C.), thus arresting the penetration of the liquid contents. This is achieved by using calcium aluminum silicates, or mixtures of calcium silicates and alumina, as the barrier material. In Column 3, lines 54 to 58 of the patent, there is a warning against the use of materials which contain mineral phases which absorb water during storage or installation, e.g. free CaO and 3CaO.SiO.sub.2. This barrier material is intended to protect the refractory lining from penetration by liquid cell contents, but apparently does not protect against penetration by solid fluorides. Moreover, since the barrier material reacts with the fluorides from the cell, disposal of this material would cause environmental problems.
U.S. Pat. No. 4,683,046 to Scharpey et. al. which issued on Jul. 28, 1987 discloses the use of various metal oxides for sealing gaps present between carbon-carbon blocks used to form the cathode in cells of the above kind, and to seal other similar joints. The sealants include calcium oxide, magnesium oxide, barium oxide, calcium fluoride, magnesium fluoride, barium fluoride, silicon oxide, iron-III-oxide, silicon carbide, titanium nitride, boron nitride, chromium-III oxide and aluminum oxide, and are used in powder form. However, these materials are "limited to the particularly vulnerable regions" and, like the Seltveit invention mentioned above, are concerned with the prevention of penetration of molten aluminum and electrolyte into the refractory lining. There is no mention of the problem caused by the penetration of solid fluorides.
U.S.S.R. patent 711,335 published on Jan. 25, 1980 in the name of the State Institute for Secondary Light Metal Research, Study and Planning, discloses the use of a layer of calciferous fibrous material (which could also be translated as "fibrous lime material") in order to prevent penetration of molten aluminum or other metal into a refractory furnace lining layer. According to the patent, molten metal penetrating the refractory lining finds the calciferous fiber layer in its path and is brought to a halt. Again, the invention is concerned with the prevention of penetration by molten materials rather than the diffusion of harmful solids. Moreover, fluorides are not present in the metal melting furnaces to which this patent relates, so there is no clear relationship between this patent and the present invention.
Despite these teachings of the prior art, there is therefore still a need for a way of preventing solid fluorides from penetration into refractory linings of aluminum reduction cells or similar high temperature metal treatment apparatus in which penetration of solid fluorides into refractory insulating layers is of concern.